I have sketched out the default playlist format on the wiki:
http://playlist.musicbrainz.org/playlist/moin.cgi/DefaultFormat
What that is is the pattern you almost always see in ad-hoc formats.
It's in ASX, KPL, SMIL, etc.
Now, there are good reasons to depart from it, but they have to be
considered departures from the obvious.
- Lucas